Keeping it short.

I can't find the permission for the command "/pex group rank "10""
Any help is appreciated.

If this is a PEX 1.x question, you'd be looking for permissions.groups.rank.<group> and the command is actually pex group <group> rank <number> with a lower number being "more powerful"/"more important" than a high number. (Found using pex user Stormbow toggle debug on my 1.7.2 server.)

For the purposes of setting up and testing servers, I always advise server owners to give themselves (and no one else, ever) full access to all of the PEX commands with pex user <user> add permissions.* e.g., pex user Stormbow add permissions.*. This way a server owner can move themselves around to any group and get the feel of the group as if they were a true member of it while setting up and troubleshooting any issues.

You're welcome.

Thank you, that's exactly what I need to know.